Add ability to mark flags as required or exclusive as a group (#1654)

This change adds two features for dealing with flags:
 - requiring flags be provided as a group (or not at all)
 - requiring flags be mutually exclusive of each other

By utilizing the flag annotations we can mark which flag groups
a flag is a part of and during the parsing process we track which
ones we have seen or not.

A flag may be a part of multiple groups. The list of flags and the
type of group (required together or exclusive) make it a unique group.

Signed-off-by: John Schnake <jschnake@vmware.com>

user_guide.md

@@ -300,6 +300,30 @@ rootCmd.PersistentFlags().StringVarP(&Region, "region", "r", "", "AWS region (re
 rootCmd.MarkPersistentFlagRequired("region")
 ```
 
+### Flag Groups
+
+If you have different flags that must be provided together (e.g. if they provide the `--username` flag they MUST provide the `--password` flag as well) then 
+Cobra can enforce that requirement:
+```go
+rootCmd.Flags().StringVarP(&u, "username", "u", "", "Username (required if password is set)")
+rootCmd.Flags().StringVarP(&pw, "password", "p", "", "Password (required if username is set)")
+rootCmd.MarkFlagsRequiredTogether("username", "password")
+``` 
+
+You can also prevent different flags from being provided together if they represent mutually 
+exclusive options such as specifying an output format as either `--json` or `--yaml` but never both:
+```go
+rootCmd.Flags().BoolVar(&u, "json", false, "Output in JSON")
+rootCmd.Flags().BoolVar(&pw, "yaml", false, "Output in YAML")
+rootCmd.MarkFlagsMutuallyExclusive("json", "yaml")
+```
+
+In both of these cases:
+  - both local and persistent flags can be used
+    - **NOTE:** the group is only enforced on commands where every flag is defined
+  - a flag may appear in multiple groups
+  - a group may contain any number of flags
+
 ## Positional and Custom Arguments
 
 Validation of positional arguments can be specified using the `Args` field